1. Field of the Invention
The present invention relates to a method and apparatus for retransmitting data based on antenna scheduling in a MIMO (Multiple Input Multiple Output) system.
2. Description of the Related Art
Recently, as the mobile communication technology rapidly develops, a technique for increasing transfer speed of data to provide a service with a more enhanced quality is considered to be important. IMT-2000, which is the third-generation wireless communication, requires transfer speed of more than 10 Mbps during stop mode and more than 384 Kbps during moving mode. Further, the post-fourth-generation wireless communication requires transfer speed of more than 155 Mbps during stop mode and more than 2 Mbps during moving mode.
To satisfy such requirements, a MIMO system has been proposed which uses a plurality of antennas provided in both transmitter and receiver sides. The MIMO technique is divided into a spatial multiplexing scheme and a spatial diversity scheme. In the spatial multiplexing scheme, a transmitter and a receiver respectively have a plurality of antennas provided therein so as to simultaneously transmit data different from each other. Without increasing the bandwidth of the system, the data can be transmitted at high speed. In the spatial diversity scheme, identical data is transmitted through a plurality of transmitting antennas so as to obtain a transmission diversity gain.
The MIMO system can significantly enhance a communication capacity and transmission/reception performance and can provide a high transmission rate, without additional frequency allocation or increase in power. Researches on the MIMO system are actively carried out. The MIMO system is considered as a core technology of the next generation communication. However, the MIMO system is vulnerable to ISI (InterSymbol interference) generated during high-speed transfer of signals and frequency selective fading of frequency caused by multiple paths. Therefore, to overcome such a disadvantage, an OFDM (Orthogonal Frequency Division Multiplexing) scheme is used together in the MIMO system.
The OFDM scheme attracts attention as a method which can satisfy high speed, a high quality, and large-volume communication required by the fourth-generation communication system. Since an OFDM signal has a plurality of sub-carriers in a frequency region and data is transmitted in parallel, the overall transfer speed is maintained as it is, and the transfer speed per sub-carrier can be reduced. Further, when a high-speed data stream is transmitted using a low-speed parallel carrier wave, a symbol interval is increased, so that the ISI is reduced. In particular, as a guard interval (GI) is used, the ISI can be almost perfectly removed. Further, as a plurality of carrier waves are used in the OFDM signal, the OFDM signal is not affected by the frequency selective fading. As two of the systems are combined, the advantage of the MIMO system is used as it is, and the disadvantage of the MIMO system can be removed by the OFDM system. This system is referred to as a MIMO-OFDM system.
Meanwhile, as an error control means for overcoming a packet transmission error which frequently occurs in a poor wireless channel when data is transmitted and received through the MIMO system, an ARQ (Automatic Repeat reQuest) protocol has been proposed. Recently, as a method for maximizing the efficiency, a hybrid ARQ (HARQ) protocol is being adopted as the standard protocol. In the HARQ protocol in which forward error correction (FEC) and the ARQ protocol are combined, error correction is performed by a receiver side through the FEC. If the error correction is failed, retransmission is carried out.
The HARQ protocol can be roughly divided into two schemes. The first scheme is referred to as a chase combining scheme. In the chase combining scheme, when an error is present in a received packet, a request for retransmission of the packet is delivered to a transmitter side. Then, the retransmitted packet and the packet having the error are combined so as to judge whether an error is present in the packet. The second scheme is referred to as an incremental redundancy (IR) scheme. In the IR scheme, a packet in which an error occurred is not retransmitted, but only additional redundancy bits are retransmitted so as to be combined with the packet where an error occurred. In this case, different redundancy bits are retransmitted at every transmission.
As the HARQ scheme is applied to the MIMO-OFDM system, an error of a packet can be effectively corrected in comparison with an existing MIMO-OFDM system, thereby enhancing the reliability of the system. In the HARQ scheme of the conventional MIMO-OFDM system, however, the packet where the error occurred is retransmitted through the same antenna. Therefore, when an error occurs in a packet transmitted through a specific antenna, an error may again occur in the packet retransmitted through the same antenna, thereby reducing the yield rate of the entire system. Further, when the retransmission is continuously requested due to some poor communication links, the entire system may be paralyzed in some cases.